What does a capacitor primarily oppose in an electrical circuit?

A capacitor primarily opposes changes in voltage within an electrical circuit. This behavior is due to the capacitor's ability to store energy in an electric field created between its plates. When a voltage is applied across a capacitor, it begins to charge, and this charging process inherently opposes any sudden changes in voltage.

The fundamental characteristic of a capacitor is its capacitance, which is defined as the ratio of the electric charge stored to the voltage across the capacitor. As the voltage changes, the capacitor either discharges or charges to maintain the stored voltage level, thus resisting rapid shifts in voltage. This is a key principle in AC circuits and filtering applications, where capacitors are utilized to smooth out voltage fluctuations and provide stability in power supplies.

In contrast, current refers to the flow of electric charge, resistance refers to the opposition to current flow in a circuit, and power relates to the rate of energy transfer. While these concepts are interconnected, they do not define the principal behavior of capacitors in the way that voltage does.